The invention relates to rail cars and more particularly to covered multiple hopper cars of the type used to handle bulk shipments of particulate materials. As a general rule, a covered hopper car can be made in a wide range of sizes to accommodate different volumes of materials. However, in the interest of safety and to prevent overloading of tracks and bridges, limits are imposed on the gross loading of a rail car and its contents which is applied to the rails. Limits are also imposed on the overall height of its load in order to keep the car's center of gravity within a prescribed limit.
The density of a particular product to be transported by a rail car can vary widely from low density plastic pellets having a density of about 28 pounds per cubic foot, to grain with a density of about 35 pounds per cubic foot and on to aggregate such as sand, gravel and cement having a density of about 90-100 pounds per cubic foot. To maximize load sizing and the efficiencies of handling, it has been common to build cars for specific commodities so that approximately 100 tons of a particular commodity can be transported in a single car. Thus, for example, one might see four-hopper cars used for handling light products, three-hopper cars used for handling grain and two-hopper cars used for handling cement, sand and gravel. Alternatively, a car can be manufactured which has smaller or larger hoppers, depending upon whether it is being designed to handle a certain weight of a high density material or a low density material. Since a hopper car can have a useful life of 40-50 years, it can and does happen that the existing stock of all types of hopper cars might periodically be out of balance with the current needs of a nation's economy.
At the present time, there is a considerable oversupply of three-hopper grain hauling cars and an undersupply of two-hopper cars for hauling aggregate. Rather than scrapping some of the three-hopper cars and then building new two-hopper cars, it would seem desirable to convert the three-hopper cars to smaller two-hopper cars which would have to be suitable for carrying aggregate.